The present invention relates generally to devices for detecting the angular position of an object, and more particularly to a device for detecting the angular position of an object relative to a preset zero position with a magnetoresistive (MR) sensor which has a constant reference axis plus contacts for supplying an electric current and is arranged in a magnetic field, where one magnetic field component (H or H.sub.g) of the magnetic field and the reference axis of the MR sensor can be rotated by an angle of rotation (.THETA. or .phi.) with respect to each other in a plane of rotation, where this angle of rotation has an unambiguous correlation with the angular position to be determined, and the electrical resistance of the MR sensor is an unambiguous function of this angle of rotation (.THETA. or .phi.). Such a device is disclosed, for example, in the Philips Technical Information Brochure TI 901228 "Properties and Applications of KMZ 10 Magnetic Field Sensors."
In one embodiment of this known device for measuring the angular position, a magnetoresistive barber-pole sensor is set up in the field of a rotationally mounted magnet.
Magnetoresistive sensors are composed in general of a thin layer of a magnetoresistive material which is magnetized in the plane of the layer. When the magnetization of the layer is rotated with respect to the direction of a measurement current flowing in the layer by a magnetic field which is to be measured, there is a change in resistance that may amount to several percent of the normal isotropic resistance and can be detected as a measurement signal. This effect is called anisotropic magnetoresistance or the anisotropic magnetoresistive effect (AMR). The customary magnetoresistive materials are ferromagnetic transition metals such as Nickel (Ni), iron (Fe), or Cobalt (Co) and alloys made with these metals. At least one rectangular strip that is made of the ferromagnetic NiFe alloy known commercially as Permalloy and is magnetized in its longitudinal direction is provided with the magnetoresistive barber-pole sensor used in the known angle measurement device. Several thin metal strips are arranged side by side on the Permalloy strip at an angle of 45.degree. to the longitudinal direction of the Permalloy strip. If voltage is now applied to the Permalloy strip in its longitudinal direction, an electric current is generated between the metal strips, where the direction of this current is essentially at an angle of .+-.45.degree. or .+-.135.degree., depending on the polarity of the voltage, to the magnetization of the Permalloy strip. An external magnetic field that is to be measured and has a component at right angles to the magnetization then rotates the magnetization of the Permalloy strip relative to the direction of the current which remains constant. This rotation causes a change in resistance that has an approximately linear dependence on the magnetic field. The characteristic curve of the resistance of such a barber-pole sensor is unambiguous and at least approximately linear for an angle range of approximately 90.degree., which may be selected between about +45.degree. to -45.degree. or about -45.degree. and +45.degree. for the angle between the magnetization and the measurement current.
The magnetic field of the rotatable magnet is thus provided both as a measurement field and as a supporting field to stabilize the sensor characteristic. Rotation of the magnet through an angle to be measured creates a change in the resistance signal of the barber-pole sensor. The measurement range of this angle-measuring device is limited to a maximum of .+-.90.degree. because the sensor has an unambiguous characteristic curve only in this angular range. Larger angles up to .+-.135.degree. can be achieved by using the supporting field of an additional magnet. Angles of up to almost .+-.180 degrees are feasible by using two sensors set up at right angles to one another and analyzing their measurement signals in the individual angle quadrants. Possible applications of this known device are for gas pedal sensors and throttle valve sensors for motor vehicles, gradient sensors, and wind direction indicators.
In another embodiment of this invention intended for use as a compass, two barber-pole sensors crossed at right angles are set up to rotate in the magnetic field of a coil whose function is to reverse the magnetism of the sensors ("Technical Information Publication TI 901228" from Philips Components).
There are known multi-layered systems that have several ferromagnetic layers arranged in a stack and separated from one another by intermediate nonmagnetic metals layers, where the magnetization of each magnetic layer is in the plane of the layer. The thickness of each layer is selected so it is considerably smaller than the mean free path of the conduction electrons. When an electrical current is applied in such a layer system, the so-called magnetoresistive effect of giant magnetic resistance (giant MR) also occurs in the individual layers in addition to the anisotropic magnetoresistive effect. This giant MR effect is due to the varying intensity, as a function of the given magnetization, of the scattering of the majority and minority conduction electrons in the volume of the layers, especially in alloys, but also at the interfaces between the ferromagnetic layers and the intermediate layers. This giant MR is an isotropic effect--in other words, it is not dependent on the direction of the current in particular, and it can be considerably greater than the anisotropic MR, with values of up to 70% of the normal isotropic resistance.
Two basic types of such giant MR multi-layered systems are known. In the first type, the ferromagnetic layers are linked together antiferromagnetically across the intermediate layers, so the magnetizations in the planes of two neighboring ferromagnetic layers are aligned antiparallel to each other without any external magnetic field. One example of this type would be the iron-chromium superlattices (Fe-Cr superlattices) with ferromagnetic layers of Fe and antiferromagnetic intermediate layers of Cr. With a properly aligned external magnetic field, the magnetization of the neighboring ferromagnetic layers is rotated against the antiferromagnetic coupling forces and is aligned in parallel. This reorientation of the magnetization by the magnetic field results in a steady decrease in the giant MR, which is a measure of the size of the magnetic field. At saturation field strength H.sub.s there is no further change in the giant MR because all the magnetizations are then aligned parallel to one another ("Physical Review Letters," vol. 61, no. 21, Nov. 21, 1988, pp. 2472-2475).
In the second type of giant MR multi-layered system, the ferromagnetic layers are separated from one another by intermediate diamagnetic or paramagnetic layers made of metal. The thickness of the intermediate layers is adjusted so the magnetic exchange coupling between the magnetizations of the ferromagnetic layers is as small as possible. The neighboring ferromagnetic fields have different coercive field strengths.
Using an exchange-decoupled giant MR layer system of this type with magnetically softer measurement layers made of Ni.sub.80 Fe.sub.20 and magnetically harder biasing layers made of Co separated from the measurement layers by intermediate layers of Cu, the resistance has already been measured as a function of the angle .THETA. between a saturation magnetic field strength H.sub.0 and a magnetic field H.sub.r rotating parallel to the planes of the layers. The size of the rotating field H.sub.r was in this case selected such that only the magnetization M.sub.1 of the measurement layers would follow the rotation of the magnetic field H.sub.r, and the magnetization M.sub.2 of the biasing layers would persist in the original alignment, which is determined by the saturation field H.sub.0. It was found that the total electrical resistance R of the layer system as a function of the angle .THETA. can be represented in good approximation by the sum of a component A.sub.1 .multidot.cos (.THETA.) for the giant MR, a component A.sub.2 .multidot.cos (2.multidot..THETA.) for the AMR, and a constant resistive component R.sub.0, where the coefficient A.sub.2 of the AMR component is positive and the coefficient A.sub.1 of the giant MR component is negative and is about 60 times larger than A.sub.2 ("Journal of Magnetism and Magnetic Materials" (North-Holland), vol. 113, 1992, pp. 79-82).
The present invention is directed to the problem of developing a device for measuring the angular position of an object as described in the introduction so that an angular measurement of at least 180.degree. is achieved, along with greater sensitivity and a larger field range.